The invention relates to a multiple pass optical amplifier and a method for amplifying electromagnetic radiation, particularly light, and especially laser light.
There are an increasing number of applications for laser systems where the power of the laser beam must be maximized, and one attractive method for reaching high power levels is to amplify a laser beam generated in a separate laser by passing it through a laser gain material such as Nd:YAG or Nd:YVO4. Many such amplifier systems have been demonstrated including straight forward single pass, and zig-zag designs. In order to maximize the energy extracted from a laser amplifier, of whatever design, multiple co-linear passes of the amplifier can be made. These multi-pass amplifier designs typically use polarization rotating devices to enable separation of the input and output laser beams. For example it is possible to obtain two passes of the amplifier using an optical polarizer and a quarter-wave plate, or using an optical isolator. To increase the efficiency of the amplifier further it is possible to obtain four co-linear passes of the gain material, for example, using an optical polarizer, a quarter-wave plate and an optical isolator, as proposed in U.S. Pat. No. 5,268,787 (McIntyre).
Although the multiple pass amplifier designs described above can in themselves increase the efficiency of optical amplifiers, this design in itself fails to address two important issues which can have a serious detrimental effect on amplifier performance.
One such issue is thermal depolarization which is significant in many solid state laser gain media. Thermal depolarization arises as a result of thermal stress due to absorption of pump or other energy. This induces stress-related birefringence in the gain material, which can cause rotation of the polarization of the amplified laser beam in the gain medium. Various techniques to compensate for thermal depolarization in laser resonators have been reported. In particular in W. A. Clarkson, N. S. Felgate, and D. C. Hanna, xe2x80x9cSimple method for compensation of thermally-induced birefringence in high-power solid-state lasersxe2x80x9d, EuroCLEO 98, a quarter-wave plate, aligned to give zero phase retardation for the favored laser polarization, is inserted into the laser cavity. Further in C. A. Denman and S. I. Libby, xe2x80x9cBirefringence Compensation using a Single Nd:YAG Rodxe2x80x9d, Advanced Solid State Lasers 1999, a 45-degree Faraday rotator is inserted into the laser cavity and used in combination with an optical polarizer. In both of these approaches the thermally induced birefringence of an initial pass is substantially cancelled in the next pass. In order to achieve this the amplified beam must experience birefringence distribution in the following pass that is, to the extent possible, the same as that in the initial pass. Usually this is done by making the beam re-trace the initial beam path. However, 4-pass amplifiers constructed in this way have the disadvantage that unwanted lasing might occur.
Apart from thermally induced birefringence another important issue, especially in the case of four-pass amplifiers, is the avoidance of unwanted lasing in the amplifier. This can occur when a stable optical cavity is formed by the mirrors in the amplifier system used to reflect the beam through its multiple passes of the gain medium. Lasing is particularly likely to occur in a 4-pass amplifier, when the thermally induced birefringence is large, and thus can rotate the polarization of the light on each pass through the gain medium to such a degree that light can be xe2x80x9ctrappedxe2x80x9d in the amplifier rather than exiting through the polarizing beamsplitter after making a pass through the system. Thus thermal depolarization in the gain medium can be significant enough to allow lasing to occur where the presence of a polarizing beamsplitter and a polarization rotating device would otherwise prevent repeated passes along the same beam path.
It is an object of the invention to provide a multiple-pass optical amplifier and a method for amplifying light where thermal birefringence is compensated. It is a further object of the invention to provide a multiple-pass optical amplifier and a method for amplifying light where undesired lasing in the amplifier is avoided.
For studying the effect of thermal-stress-induced birefringence in a laser gain medium, the effect of the birefringence on a linearly polarized input beam can be considered. In passing through the birefringent gain medium (or any other birefringent component), the linearly polarized input beam is transformed into an elliptically polarized beam. Here we note that the direction of rotation of the polarization (i.e., left-handed or fight-handed) depends on the orientation of the fast and slow birefringent axes in the gain medium relative to the input polarization. Hence if the fast and slow birefringent axes are exchanged, then the direction of rotation is reversed. In order to compensate for the thermal-stress-induced birefringence, the elliptically polarized beam should be transformed back into one which is again linearly polarized. This could, at least in principle, be achieved by passing the beam through a device which exhibits a birefringence which is exactly equal and opposite to that of the gain medium. This means that at every point the two orthogonal birefringent axes are exchanged. However, the birefringence (or inverse birefringence) distribution in a typical laser rod is a function of position in the rod; it depends on the thermal load, the rod type etc. Therefore, it is not easily reproduced, and thus it is not generally possible to use such a separate device.
Instead of introducing a separate device in which the birefringent axes are exchanged compared to those in the gain medium, the invention exploits the birefringence distribution already present in the gain medium. This is done by modifying the polarization state of the elliptically polarized beam in a way (or ways) which is equivalent to exchanging the birefringent axes in the gain medium, and by passing the beam for a second time through the gain medium. In other words, the polarization state of the beam is modified so that it is the same as that which would have been obtained if the birefringent axes of the gain medium had been exchanged for the first pass.
According to the invention, the multiple pass optical amplifier for an incident light beam comprises:
an optical gain material;
means for reflecting a light beam which has passed once through said gain material back into said gain material; means for modifying the polarization state of a light beam after passing through said gain material for a first time and before passing through said gain material for a second time with respect to two orthogonal axes in a way which is equivalent to exchanging said two orthogonal axes; and
means for separating a light beam which has passed twice through said gain material from said incident light beam.
According to the invention, the method for amplifying an incident light beam comprises the steps of:
passing said incident light beam through an optical gain material;
reflecting the light beam which has passed once through said gain material back into said gain material;
modifying the polarization state of the light beam after passing through said gain material for a first time and before passing through said gain material for a second time with respect to two orthogonal axes in a way which is equivalent to exchanging said two orthogonal axes;
passing said incident light beam through said gain material for a second time; and
separating the light beam which has passed twice through said gain material from said incident light beam.
A first exemplified embodiment of a 2-pass amplifier according to the invention rotates all polarization components of the elliptically polarized beam through 90xc2x0. This is equivalent to rotating the birefringent axes of the gain medium through 90xc2x0, which has the same effect as exchanging the two orthogonal axes of the gain medium. The 2-pass amplifier comprises a polarization rotator and a reflecting element which are both arranged behind the gain medium. The polarization rotator rotates the polarization plane of the light through 45xc2x0 and is preferably a Faraday rotator. It rotates all polarization components of an optical beam in the same direction and by the same amount. The elliptically polarized beam passes two times through the polarization rotator, before and after its reflection. Therefore, it experiences a total polarization rotation of 90xc2x0. The polarization component of the elliptical beam which experienced one birefringent axis on the first pass of the gain medium will now experience the other on the second pass, and vice-versa.
A second exemplified embodiment of a 2-pass amplifier according to the invention reverses the direction of the rotation of the elliptical polarization of the beam and leaves the polarization otherwise unchanged. Thus the resulting polarization is the same as would be obtained if the two orthogonal birefringent axes in the gain medium had been exchanged for the first pass, and birefringence compensation will be achieved in the second pass of the laser rod. The 2-pass amplifier comprises a quarter-wave plate and a reflecting element which are both arranged behind the gain medium. The quarter-wave plate is oriented in such a way that it would not change the polarization of the linearly polarized beam if there was no birefringence in the gain medium. However, with thermally induced birefringence, light which passed once through the gain medium is generally elliptically polarized. Over two passes the quarter-wave plate generates a half-wave retardation and hence reverses the direction or the rotation of the elliptical polarization induced by thermal birefringence.
In the embodiment with the quarter-wave plate, complete birefringence compensation is only achieved for those components where the wave plate lies along the principle axis of the polarization ellipse. This will not be the case for all birefringence orientation in the gain medium. However, significant compensation of the total birefringence effects can still be achieved. For example in the case of a cylindrical laser rod in which, due to cylindrically symmetric thermal loading, the birefringent axes are basically radial and tangential, the quarter-wave plate can be aligned parallel to the input polarization. In this case, the effect of birefringence will be compensated at all points lying on lines at 0xc2x0, 45xc2x0, 90xc2x0, 135xc2x0 (and 180xc2x0, 225xc2x0, 270xc2x0, 315xc2x0) from the input polarization and through the axis of symmetry. At these points, it can be shown that one axis or the polarization ellipse will be parallel to the input polarization. (In fact, at points lying on lines at 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0 from the input polarization and through the axis of symmetry no birefringence compensation is required because the birefringent axes here are either parallel or perpendicular to the input polarization and so no depolarization occurs.).
It should be noted that the birefringence in the gain medium varies with the position in the gain medium. To achieve good polarization compensation, care must therefore be taken to ensure that any particular part of the beam being amplified experiences the same birefringence in the second pass as in the first pass. Ideally it should follow the same path (possibly in the opposite direction). In many cases, the birefringence in a gain medium exhibits symmetry, and this may be exploited when trying to fulfill the above requirement; possibly the exactly same path need not be followed.
In the four pass configuration the thermal birefringence experienced in the third pass is compensated for by that of the fourth pass, in the same way as that of the first pass is compensated in the second.
In order to prevent unwanted lasing in an amplifier it is possible to design the system so that no stable optical cavity is formed. In this case there is no possible laser mode which would reproduce itself with each passage through the potential laser cavity. However in general this means that the laser beam being amplified also fails to follow the same path through the gain medium in consecutive passes, and therefore that the thermal depolarization measures described above become ineffective or less effective. It turns out that in general all amplifier designs in which the optical path of the amplified beam exactly retraces that of the previous path a stable optical cavity tends to be formed (or one which is very close to stability) and unwanted lasing can occur.
Unwanted lasing in the amplifier can be avoided by making the beam follow a non-reciprocal but symmetrical beam path through the amplifier. In this case it is possible to design the optical system so that no stable optical cavity is formed. At the same time, for the case of an amplifier which exhibits symmetry about a plane the normal of which is parallel to the optical axis, the thermally induced birefringence in the gain medium can remain fully or almost fully compensated for as the thermally induced birefringence distribution exhibits the same symmetry.
According to the invention, a further embodiment of a multiple pass optical amplifier for an incident light beam comprises:
an optical gain material;
first means for reflecting a light beam which has passed through said gain material back into said gain material;
first means for separating a light beam which has twice passed through said gain material from said incident light beam;
second means for reflecting a light beam which has twice passed through said gain material back into said gain material;
second means for separating a light beam which has four times passed through said gain material from said incident light beam;
whereby all components of the amplifier are designed and mutually arranged in such a way that
upon passing said gain material for a second time, the path of the light beam is non-reciprocal but symmetrical about a center plane of said gain material, the normal of which is parallel to the optical axis, with respect to the path of a light beam passing through said gain material for a first time, and
upon passing said gain material for a fourth time, the path of the light beam is non-reciprocal but symmetrical about a center plane of said gain material, the normal of which is parallel to the optical axis, with respect to the path of a light beam passing through said gain material for a third time.
According to the invention, a further embodiment of the method for amplifying an incident light beam comprises the steps of:
passing said incident light beam through an optical gain material;
reflecting the light beam which has passed through said gain material back into said gain material;
passing the light beam through said optical gain material for a second time, whereby the path of the light beam is non-reciprocal but symmetrical about a center plane of said gain material, the normal of which is parallel to the optical axis, with respect to the path of the light beam passing through said gain material for a first time, and
separating the light beam which has twice passed through said gain material from said incident light beam;
reflecting the light beam which has twice passed through said gain material back into said gain material;
passing the light beam through said optical gain material for a third time;
reflecting the light beam which has passed through said gain material back into said gain material;
passing the light beam through said optical gain material for a fourth time, whereby the path of the light beam is non-reciprocal but symmetrical about a center plane of said gain material, the normal of which is parallel to the optical axis, with respect to the path of a light beam passing through said gain material for a third time; and
separating the light beam which has four times passed through said gain material from said incident light beam.